U.S. patent application number 11/598377 was filed with the patent office on 2007-06-14 for method and apparatus for drawing a low liquidus viscosity glass.
Invention is credited to Olus Naili Boratav, Frank Thomas Coppola, Andrey V. Filippov, Allan Mark Fredholm, Bruno Le Gallic, Michael Brian Gildea, George Clinton Shay.
Application Number | 20070130994 11/598377 |
Document ID | / |
Family ID | 38123386 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070130994 |
Kind Code |
A1 |
Boratav; Olus Naili ; et
al. |
June 14, 2007 |
Method and apparatus for drawing a low liquidus viscosity glass
Abstract
A method of a drawing a glass ribbon from molten glass sheet via
a downdraw process by creating a temperature drop across a
thickness of the molten glass flowing over forming surfaces of a
forming wedge. The forming wedge includes an electrically
conductive material for heating the glass above the root.
Inventors: |
Boratav; Olus Naili;
(Ithaca, NY) ; Coppola; Frank Thomas; (Horseheads,
NY) ; Filippov; Andrey V.; (Painted Post, NY)
; Fredholm; Allan Mark; (Hericy, FR) ; Gildea;
Michael Brian; (Painted Post, NY) ; Gallic; Bruno
Le; (Fontainbleau, FR) ; Shay; George Clinton;
(Moneta, VA) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
38123386 |
Appl. No.: |
11/598377 |
Filed: |
November 13, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60748887 |
Dec 8, 2005 |
|
|
|
Current U.S.
Class: |
65/29.21 ;
65/162; 65/324; 65/90 |
Current CPC
Class: |
C03B 17/064
20130101 |
Class at
Publication: |
065/029.21 ;
065/090; 065/324; 065/162 |
International
Class: |
C03B 18/22 20060101
C03B018/22; C03B 13/00 20060101 C03B013/00; C03B 5/24 20060101
C03B005/24 |
Claims
1. A method of forming a glass sheet comprising: flowing a molten
glass having a liquidus viscosity less than about 100,000 poise
over a forming wedge, the forming wedge comprising an electrically
conductive member and forming surfaces which converge at an apex;
heating the forming wedge proximate the apex by flowing a current
through the electrically conductive member; and wherein the heating
is effective to produce a temperature drop through a thickness of
the molten glass greater than about 20.degree. C.
2. The method according to claim 1 wherein the temperature drop is
greater than about 40.degree. C.
3. The method according to claim 1 further comprising cooling a
surface of the molten glass.
4. The method according to claim 1 wherein a strain point of the
molten glass is at least about 665.degree. C.
5. The method according to claim 1 wherein the liquidus viscosity
is less than about 80,000 poise.
6. The method according to claim 1 wherein an average viscosity of
the glass at the apex is greater than about 100,000 poise.
7. A method of forming a glass sheet comprising flowing a molten
glass having a liquidus viscosity less than about 100,000 poise
over a forming wedge to form a glass ribbon, the forming wedge
comprising an electrically conductive member and forming surfaces
which converge at an apex,; heating the forming wedge proximate the
apex by flowing a current through the electrically conductive
member; cooling a surface of the molten glass; and wherein the
heating and cooling is effective to produce a temperature drop
through a thickness of the molten glass greater than about
20.degree. C.
8. The method according to claim 7 wherein an average viscosity of
the glass at the apex is greater than about 100,000 poise.
9. The method according to claim 7 wherein a strain point of the
molten glass is at least about 665.degree. C.
10. The method according to claim 7 wherein the temperature drop is
greater than about 40.degree. C.
11. An apparatus for forming a glass sheet comprising: a forming
wedge having an electrically conductive member for heating a molten
glass flowing over the forming wedge by flowing a current through
the electrically conductive member.
12. The apparatus according to claim 11 wherein the electrically
conductive member comprises a cladding over at least a portion of
the forming wedge.
13. The apparatus according to claim 1 I wherein the electrically
conductive member comprises a cap member in abutment with the
forming wedge.
14. The apparatus according to claim 11 wherein the cap member
contains a void.
15. The apparatus according to claim 11 wherein the electrically
conductive member comprises a keel member extending from the
forming wedge.
16. The apparatus according to claim 11 wherein at least a portion
of the electrically conductive member is embedded within the
forming wedge.
17. The apparatus according to claim 1 1 wherein substantially all
of the electrically conductive member is embedded within the
forming wedge.
18. The apparatus according to claim 11 wherein the glass contacts
the electrically conductive member.
19. The apparatus according to claim 11 further comprising a
cooling element for cooling a surface of the molten glass.
20. The apparatus according to claim 18 wherein the cooling element
is disposed opposite the electrically conductive element such that
the molten glass flows between the electrically conductive member
and the cooling element.
Description
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn. 119(e) of U.S. Provisional Application Ser. No.
60/748887 filed on Dec. 8, 2005, the contents of which are
incorporated herein in their entirety by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention is directed to a method and apparatus for
forming a glass sheet, and in particular, forming a temperature
drop across a thickness of molten glass flowing over a forming
wedge to enable the drawing of low viscosity glasses via a downdraw
process.
[0004] 2. Technical Background
[0005] Glass display panels in the form of liquid crystal displays
(LCDs) are being used in an increasing variety of
applications--from hand-held personal data assistants (PDAs) to
computer monitors to television displays. These applications
require glass sheets which have pristine, defect-free surfaces.
LCDs are comprised of at least several thin sheets of glass which
are sealed together to form an envelope. It is highly desirable
that the glass sheets which comprise these displays do not deform
when cut, thereby maintaining the proper registration, or
alignment, between the elements. Residual stress which may be
frozen into the glass, if relieved by cutting the glass into
smaller portions, may result in deformation of the glass, and a
loss of proper registration.
[0006] Typically, LCDs are of the amorphous silicon (.alpha.-Si)
thin film transistor (TFT) or, more recently,
polycrystalline-silicon (.rho.-Si or poly-Si) TFT type. It is
possible, using .rho.-Si processing, to build the display drive
circuitry directly on the glass substrate. By contrast, .alpha.-Si
requires discrete driver chips that must be attached to the display
periphery utilizing integrated circuit packaging techniques.
[0007] The evolution from .alpha.-Si to .rho.-Si has presented a
major challenge to the use of a glass substrate. Poly-Si coatings
require much higher processing temperatures than do .alpha.-Si, in
the range of 600-700.degree.. Thus, the glass substrate must be
thermally stable when heated to such temperatures. Thermal
stability (i.e. thermal compaction or shrinkage) is dependent upon
both the inherent viscous nature of a particular glass composition
(as indicated by its strain point) and the thermal history of the
glass sheet as determined by the manufacturing process. High
temperature processing, such as required by poly-Si TFTs, may
require long annealing times for the glass substrate to ensure low
compaction.
[0008] One method of producing glass for optical displays is by an
overflow downdraw process. U.S. Patent Nos. 3,338,696 and 3,682,609
(Dockerty), which are incorporated in their entirety herein by
reference, disclose a fusion downdraw process which includes
flowing a molten glass over the edges, or weirs, of a forming
wedge, commonly referred to as an isopipe. The molten glass flows
over converging forming surfaces of the isopipe, and the separate
flows reunite at the apex, or root, where the two converging
forming surfaces meet, to form a glass ribbon, or sheet. Thus, the
glass which has been in contact with the forming surfaces is
located in the inner portion of the glass sheet, and the exterior
surfaces of the glass sheet are contact-free. Pulling rolls are
placed downstream of the isopipe root and capture edge portions of
the ribbon to adjust the rate at which the ribbon leaves the
isopipe, and thus help determine the thickness of the finished
sheet. The contacted edge portions are later removed from the
finished glass sheet. As the glass ribbon descends from the root of
the isopipe past the pulling rolls, it cools to form a solid,
elastic glass ribbon, which may then be cut to form smaller sheets
of glass
[0009] A current limitation of the fusion draw process is related
to the material properties of the glass to be processed. It is well
known that when a glass composition initially in the molten state
is exposed to a sufficiently low temperature for a significant
amount of time, the development of crystal phases will initiate.
The temperature where these crystal phases start to develop is
known as the liquidus temperature. The crystallization point may
also be cast in terms of the liquidus viscosity, which is the
viscosity of the particular glass composition at the liquidus
temperature.
[0010] As known and currently practiced, when using the fusion draw
process it is necessary to maintain the viscosity of the glass at
the location where it leaves the isopipe at a value greater than
about 100,000 poise, more typically greater than about 130,000
poise. If the glass has a viscosity lower than about 100,000 poise,
the quality of the sheet degrades, e.g. in terms of maintaining the
sheet flatness and controlling the thickness of the sheet across
its width, and glass sheet thus produced is no longer suitable for
display applications.
[0011] According to current practice, if a glass composition which
has a liquidus viscosity of less than about 100,000 poise is
processed under conditions such that the dimensional quality of the
glass sheet would be adequate, devitrification may develop on the
isopipe and lead to crystalline particulate in the glass sheets.
This is not acceptable for display glass applications.
SUMMARY
[0012] In an embodiment according to the present invention, a
method of forming a glass sheet is disclosed comprising flowing a
molten glass having a liquidus viscosity less than about 100,000
poise over a forming wedge to form a glass ribbon, the forming
wedge comprising forming surfaces which converge at an apex,
heating the apex by flowing a current through an electrically
conductive material comprising the forming wedge, cooling a surface
of the molten glass, and wherein the heating and cooling is
sufficient to form a temperature drop across a thickness of the
molten glass adjacent the forming surfaces greater than about
20.degree. C.
[0013] In another embodiment according to the present invention, an
apparatus for forming a glass sheet is provided comprising a
forming wedge comprising forming surfaces which converge at an
apex, and an electrically conductive material for heating a molten
glass flowing over the forming surfaces by flowing a current
through the material. Preferably, the glass is heated proximate the
apex.
[0014] The invention will be understood more easily and other
objects, characteristics, details and advantages thereof will
become more clearly apparent in the course of the following
explanatory description, which is given, without in any way
implying a limitation, with reference to the attached Figures. It
is intended that all such additional features and advantages be
included within this description, be within the scope of the
present invention, and be protected by the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a perspective, partial cross sectional view of a
fusion downdraw apparatus.
[0016] FIG. 2 is a cross sectional view of a portion of the forming
wedge of FIG. 1, showing the glass flow over the converging
surfaces thereof.
[0017] FIG. 3 is a cross sectional view of a portion of the forming
wedge of FIG. 1 showing an electrically conductive material
embedded within the forming wedge proximate the forming wedge apex
and used to heat the glass flowing over the forming surfaces.
[0018] FIG. 4 is a cross sectional view of a portion of the forming
wedge of FIG. 1 showing an electrically conductive material
cladding disposed over the forming wedge proximate the forming
wedge apex and used to heat the glass flowing over the forming
surfaces.
[0019] FIG. 5 is a cross sectional view of a portion of the forming
wedge of FIG. 1 showing the apex of the forming wedge formed by a
cap comprised of electrically conductive material and used to heat
the glass flowing over the forming surfaces.
[0020] FIG. 6 is a cross sectional view of a portion of the forming
wedge of FIG. 5 showing the apex of the forming wedge formed by a
cap comprised of electrically conductive material and containing a
void.
[0021] FIG. 7 is a cross sectional view of a portion of the forming
wedge of FIG. 1 showing an the forming wedge with a keel member
formed from an electrically conductive material and used to heat
the glass.
[0022] FIG. 8 is a cross sectional view of a portion of the forming
wedge of FIG. 1 showing a cladding-type electrically conductive
element comprising the forming wedge, and a device located opposite
a forming surface for cooling the glass flowing over the forming
surface.
DETAILED DESCRIPTION
[0023] In the following detailed description, for purposes of
explanation and not limitation, example embodiments disclosing
specific details are set forth to provide a thorough understanding
of the present invention. However, it will be apparent to one
having ordinary skill in the art, having had the benefit of the
present disclosure, that the present invention may be practiced in
other embodiments that depart from the specific details disclosed
herein. Moreover, descriptions of well-known devices, methods and
materials may be omitted so as not to obscure the description of
the present invention. Finally, wherever applicable, like reference
numerals refer to like elements.
[0024] As used herein, a downdraw glass sheet manufacturing process
refers to any form of glass sheet manufacturing processes in which
glass sheets are formed while viscous glass is drawn in a downward
direction. In a fusion downdraw forming process particularly,
molten glass flows into a trough, then overflows and runs down both
sides of a pipe or forming wedge. The two flows fuse together at
what is known as the root (where the pipe ends and the two overflow
portions of glass rejoin), and the combined flow is drawn downward
until cool.
[0025] The overflow glass sheet manufacturing process can be
described with the help of FIG. 1, wherein forming wedge 10
includes an upwardly open channel 12 bounded on its longitudinal
sides by wall portions 14, which terminate at their upper extent in
opposed longitudinally-extending overflow lips or weirs 16. The
weirs 16 communicate with opposed outer sheet forming surfaces of
forming wedge 10. As shown, forming wedge 10 is provided with a
pair of substantially vertical forming surface portions 18 which
communicate with weirs 16, and a pair of downwardly inclined and
converging surface portions 20 which terminate at a substantially
horizontal lower apex 22 forming a straight, glass draw line.
[0026] Molten glass 24 is fed into channel 12 by means of delivery
passage 26 communicating with channel 12. The feed into channel 12
may be single ended or, if desired, double ended. A pair of
restricting dams 28 are provided above overflow weirs 16 adjacent
each end of channel 12 to direct the overflow of the free surface
30 of molten glass 24 over overflow weirs 16 as separate streams,
and down opposed forming surface portions 18, 20 to root 22 where
the separate streams, shown in chain lines, converge to form a
sheet, or ribbon, of virgin-surfaced glass 32.
[0027] In the fusion process, a pulling device in the form of
pulling rolls or rollers 34 are placed downstream of forming wedge
root 22 and are used to adjust the rate at which the formed ribbon
of glass leaves the converging forming surfaces and thus help
determine the nominal thickness of the finished sheet. Suitable
pulling rolls are described, for example, in published U.S. patent
application Ser. No. 2003/0181302. The pulling rolls are preferably
designed to contact the glass ribbon at its outer edges. The glass
edge portions 36 which are contacted by the pulling rolls are later
discarded from the sheet.
[0028] One advantage to the fusion forming process described above
is that the ribbon can be formed without the external ribbon
surfaces contacting the forming surfaces. This provides for smooth,
contaminant-free ribbon surfaces. In addition, this technique is
capable of forming very flat and thin glass sheets to very high
tolerances. However, other glass sheet forming techniques may also
benefit from the present invention, including, but not limited to,
slot draw and single-sided overflow downdraw forming techniques. In
the slot draw technique, molten glass flows into a trough having a
machined slot in the bottom. The glass is pulled down through the
slot, thereby forming a ribbon of glass. The quality of the glass
is obviously dependent, among other things, on the accuracy of the
machined slot.
[0029] The fusion draw process is capable of producing very high
quality glass sheets. One of its limitations, however, is that high
quality sheets can only be obtained if the glass viscosity on the
forming surfaces 18, 20, and particularly at the point where the
glass leaves the forming wedge (i.e. the apex or root), is kept at
a sufficiently high viscosity. As molten glass 24 overflows the
forming wedge weirs, it is a relatively high temperature-low
viscosity glass, on the order of about 50,000 poise for some
glasses used in the display industry. As the glass flows down
forming surfaces 18, 20, it cools and the viscosity increases,
until at the apex of the converging forming surfaces the viscosity
of the glass is sufficiently high that a commercially viable glass
sheet may be drawn for a given draw rate and glass thickness, and
assuming an appropriate glass composition. It is currently believed
that a viscosity at the apex not lower than about 100,000 poise is
required to produce quality glass with current glass compositions
and processing parameters.
[0030] In spite of the success of current downdraw forming
techniques, and in particular the fusion downdraw process,
commercial requirements are driving a need for high strain point
glass in display applications in order to provide for minimal
post-forming dimensional changes in the glass (compaction), such as
may occur during subsequent processing by customers. For the family
of glass compositions currently in use or under consideration for
display applications, a high strain point typically incurs also a
high liquidus temperature (i.e. a low liquidus viscosity). To
ensure that crystallization does not occur, the temperature of the
glass should be maintained above the liquidus temperature. If the
residence time of glass below the liquidus temperature is too long,
the glass may begin to crystallize. However, operation of the draw
process at temperatures higher than the liquidus temperature may
result in a viscosity at the forming wedge apex which may make
drawing of the glass difficult by creating, for example, warp in
the drawn glass. Thus, on one hand a high forming temperature may
be needed to avoid operation below the liquidus temperature, while
on the other hand the high temperature may preclude successful
forming of the glass into a sheet using a fusion downdraw glass
manufacturing method. These competing requirements have heretofore
limited the range of suitable glass compositions which may be used
in a fusion downdraw process.
[0031] Unfortunately, the latitude to significantly change
downstream processing steps to accommodate a low liquidus viscosity
glass (high liquidus temperature) is limited. For example,
increasing the flow rate to accommodate a lower viscosity glass at
the root requires an accompanying increase in the draw rate, which
then requires a corresponding increase in glass handling capability
downstream. Such changes can lead to considerable capital expenses,
and depending upon space restrictions, may not even be possible at
a given facility. The ability to draw glass compositions with
liquidus viscosities below about 100,000 poise without significant
changes to downstream processing methods would therefore have great
value by opening the fusion method to a range of new and
potentially useful glass compositions. For example, glass
compositions having a strain point higher than about 665.degree. C.
may be useful for certain display glass applications, such as in
the .rho.-Si deposition process where reduced compaction is needed.
If, for instance, a glass having a strain point of at least about
665.degree. C. strain point (e.g. 750.degree. C.) is required, no
fuision-formable glass composition has been identified to date
which may be fusion drawn into an acceptable drawn glass sheet
without modifying downstream processes.
[0032] In an embodiment of the present invention, the molten glass
flowing over the forming surfaces may be locally cooled above the
apex of the forming wedge while simultaneously heating the forming
surfaces beneath the flowing glass to achieve the required glass
viscosity at the forming wedge apex.
[0033] It has been found that in a conventional fusion downdraw
process the glass flowing over the forming surfaces is relatively
homogeneous in temperature, varying by less than about 10.degree.C.
from the wedge-glass interface, through thickness T of the glass,
to the glass-air interface. This temperature variation may, in some
cases, be less than about 5.degree. C. That is, the layer 38
adjacent the forming surfaces in FIG. 2 loses heat at a rate
approximately equal to the heat loss of layer 40, such that a
temperature t.sub.1 of glass layer 100 is approximately the same as
the temperature t.sub.2 of glass layer 40. (For simplicity, the
flow of molten glass 24 in FIG. 5 is depicted as two layers at two
different temperatures, t.sub.1, and t.sub.2. In actuality, the
flow of glass over the forming surfaces and the temperature field
.DELTA.t, are continuums in that they vary continuously as one
progresses from the forming wedge to the outside surface of the
glass. Nevertheless, it is useful to describe the flow in terms of
discrete layers.) Moreover, while the residence time for the
portion of glass layer 38 in direct contact with the forming
surfaces has been determined to be on the order of days, the
residence time for glass layer 40 opposite the forming surfaces
(i.e. at the glass-air interface) is typically less than an hour.
The long residence time at the wedge-glass interface can be
responsible for devitrification of the glass if the temperature of
the glass falls below the liquidus temperature. This becomes
especially troublesome in the case of high liquidus temperature
glasses as previously described: the long residence time at the
surface of the forming wedge, coupled with a glass temperature
which cools below the liquidus temperature as the glass descends
the forming surfaces, may result in nucleation and crystal growth
in the glass. The use of a high liquidus temperature glass can,
however, be facilitated by heating the forming wedge, particularly
along the lower portions of the forming surfaces near the apex. In
this way, the temperature of the long residence time glass in
contact with the forming wedge proximate the root can be maintained
at a temperature above the liquidus temperature of the glass, while
the average viscosity of the glass at the root or apex is
maintained approximately equal to or greater than the currently
acceptable lower viscosity limit for fusion drawing due to the
cooler outer layers of the glass flow, e.g. greater than about
100,000 poise. It should be noted that FIG. 2 depicts only a
portion of the forming wedge, and, in a fusion downdraw process, it
is desirable that the forming wedge conditions be substantially
symmetrical.
[0034] Heating of the forming wedge may be accomplished by
providing a heating element or elements in or on the forming wedge
in the vicinity of where the two flows of glass join. Heating of
the forming wedge works to maintain the high residence-time glass
above the liquidus temperature of the glass and preventing
crystallization. External cooling of the glass near the glass-air
interface, in the same region of the forming wedge where wedge
heating occurs helps ensure that the average glass viscosity at the
apex is sufficiently high to allow proper drawing of glass
consistent with a predetermined draw rate and glass thickness.
Thus, a large temperature variation .DELTA.t (=t.sub.1-t.sub.2) is
developed through the thickness of molten glass flowing over the
forming surfaces. Preferably, .DELTA.t is greater than at least
about 20.degree. C., preferably greater than about 30.degree. C.,
preferably at least about 40.degree. C., and preferably at least
about 50.degree. C.
[0035] In one embodiment according to the present invention, an
electrically conductive member may be incorporated into forming
wedge 10 proximate apex 22 and an electrical current flowed through
the member. The current flow through member 42 generates heat which
heats the forming wedge, thereby heating the glass flow in contact
with the forming wedge. As illustrated by FIG. 3, substantially all
of electrically conductive member 42 is embedded within forming
wedge 10 and, advantageously, therefore, not in contact with the
glass. By substantially all what is meant is that there would
obviously need to be electrical contact between the electrically
conductive member and the electrical power supply. Connection
between the member and the power supply, such as with electrical
cables, is more easily accomplished outside of the forming wedge.
However, what small portion of the electrically conductive member
(e.g. heating element) residing outside the forming wedge makes
only a negligible contribution to heating of the glass, if any.
Because conductive member 42 does not contact the glass, the
electrically conductive member may be formed from an electrically
conductive material which need not necessarily be compatible with
the glass chemistry. Consequently, member 42 may be comprised of,
for example, coils, bars, wires or other suitable shape having an
electrical connection with a power source (not shown) and capable
of heating the forming wedge apex area to the necessary
temperature. Preferably, member 42 comprises fine wires, as these
have the least amount of impact on the physical integrity of the
forming wedge.
[0036] In another embodiment, forming wedge 10 includes an
electrically conductive member in the form of a cladding comprising
a suitable electrically conductive refractory material, preferably
a refractory metal. A suitable refractory material is one which is
compatible with the glass chemistry, and therefore not prone to
decomposition or leaching when exposed to the glass, and resistant
to the high temperatures experienced by the forming wedge. Suitable
refractory materials are preferably members of the platinum group
metals, such as platinum, rhodium, a platinum-rhodium alloy, or the
like. FIG. 4 illustrates a portion of forming wedge 10 having an
electrically conductive cladding member 44 which covers apex 22,
and the portion of converging forming surfaces 20 adjacent apex 22.
Cladding 44 itself comprises converging forming surfaces 46 which
converge at apex 48. Glass which flows over weirs 16 and down
forming surfaces 18, 20, intercept and flow over cladding surfaces
46, then converge and join at apex 48 to form glass ribbon 32. FIG.
4 depicts one method of securing cladding 44 through the use of a
lip or tab 50 formed on an inside surface of the cladding which
mates with a corresponding groove formed in each converging forming
surface 20. Of course, other methods of securing cladding 44 to
forming wedge 10 as are known in the art may be used. For example,
if the cladding material is sufficiently stiff, the cladding may be
supported only at the ends of the forming wedge. However, it is
desirable that the surface of cladding 44 be free of gaps,
protrusions or other surface features which may disrupt the flow of
glass over the surface of the cladding. Cladding 44 may then be
heated by flowing a current through the cladding.
[0037] In another embodiment according to the present invention, as
shown in FIG. 5, apex 22 may be replaced altogether with an
electrically conductive member comprising, as above, a platinum
group metal such as platinum, rhodium, platinum-rhodium alloy, or
the like. As shown in FIG. 5, apex 22 of forming wedge 10 has been
replaced with electrically conductive cap 52. Cap 52 comprises
converging forming surfaces 54 which intersect at apex 56, and
which, when cap 52 has been fitted to forming wedge 10, extend from
forming surfaces 20. Advantageously, cap 52 may be fitted to
forming wedge 10 such that the intersection of converging forming
surfaces 20 and cap 52 present a continuous planar surface (as to
surfaces 20 and 54) to the flowing glass, thereby minimizing or
eliminating flow discontinuities in the transition from converging
forming surfaces 20 to cap 52. As in the previous embodiment, cap
52 may be secured to forming wedge 10 by a variety of methods which
do not form a disruptive surface on the cap. As shown in FIG. 5,
cap 52 is secured to forming wedge 10 through a dovetail joint 58.
However, cap 52 may be sufficiently rigid that the cap need only be
supported at the ends thereof. As before, cap 52 is heated by
flowing a current through the cap sufficient to heat the cap to a
desired temperature. The amount of current, and desired
temperature, are a function of the glass temperature-viscosity
relationship, among other factors, and are easily determined by one
having ordinary skill in the art. Cap 52 may be a solid feature, or
cap 52 may be hollow (i.e. contain a void), as shown in FIG. 6.
Void 60 within cap 52, as illustrated in FIG. 6 may contain an
insulating material. Void 60 may be covered by cover 62, which may
also be electrically conductive.
[0038] In still another embodiment of the apparatus according to
the present invention wherein a lower portion of forming wedge 10
is shown wherein apex 22 has been replaced with an electrically
conductive keel member 64. As before, keel member 64 is preferably
comprised of a platinum group metal, or alloy thereof. At least a
portion 66 of keel member 64 is embedded in forming wedge 10 at the
line where converging forming surfaces 20 converge (i.e. apex 22),
and another portion 68 extends outward (downward) from forming
wedge 10. Keel member 64 further comprises converging forming
surfaces 70 which converge at apex 72. Converging forming surfaces
70 may intersect with forming surfaces 20, or, as shown in FIG. 7,
converging forming surfaces 70 may be non-intersecting with forming
surfaces 20.
[0039] As described previously, various embodiments of electrically
conductive heating members, either cladding, embedded heaters,
caps, keels or other heating members comprising forming wedge 10
may be used to develop a temperature drop through a thickness of
the glass, preferably at or near the apex of the forming wedge. In
some embodiments it may be necessary to also cool the surface of
the glass to produce the desired temperature drop.
[0040] For example, FIG. 8 illustrates a portion of forming wedge
10 comprising clad-style heating member 44. Shown opposite the
cladding is cooling element 74 for cooling the surface of molten
glass 24. Cooling element 74 may be any cooling device suitable for
creating a temperature variation greater than about 20.degree. C.
through the glass thickness opposite the cooling element when used
in conjunction with any of the heating elements described herein.
For example, cooling element or device 74 may be comprised of
tubing extending along the length of the forming wedge through
which a cooling fluid is flowed. Diffusion member 76 may be
disposed between the cooling element and the surface of the glass
to provide for more even heat extraction from the surface of the
glass along the length of forming wedge 10. The diffusion member
may be a simple metal plate, or an enclosure, disposed between the
surface of the glass and the cooling element. The cooling device
described above and depicted in FIG. 8 may be used in conjunction
with any of the embodiments disclosed herein.
[0041] It should be emphasized that the above-described embodiments
of the present invention, particularly any "preferred" embodiments,
are merely possible examples of implementations, merely set forth
for a clear understanding of the principles of the invention. Many
variations and modifications may be made to the above-described
embodiments of the invention without departing substantially from
the spirit and principles of the invention. All such modifications
and variations are intended to be included herein within the scope
of this disclosure and the present invention and protected by the
following claims.
* * * * *